Many curing devices for molded products use an expansion vessel, also known as a curing bladder. In a curing device, most expansion vessels are pressurized with a fluid heat source, such as steam. The fluid heat source causes the expansion vessel to expand, pushing the product tightly against the mold. Further, the heat from the fluid heat source helps to at least partially cure the molded product.
Problems occur when a defect, such as a pinhole leak, forms in the expansion vessel. In the tire industry, pinhole leaks in expansion vessels on curing devices are one of the industry's largest causes of tire scrap. Pinhole leaks hi expansion vessels cost tire manufacturers millions of dollars per year. A pinhole leak allows high temperature steam to contact the innerliner of the tire. One possible result from this contact is an undercured area in the innerliner. Another possible result is the permeation of steam into the ply area of the tire causing a blister that can result in the separation of the plies. Currently, the first inspection of a tire for the effects of pinhole leaks occurs in the final finish area of the tire plant, minutes after the tire leaves the curing device. If a tire in final finish is found to be defective as a result of a pinhole leak, then up to ten additional tires may have been cured with the same defective expansion vessel prior to discovery of the problem. Generally, all of these tires must be scrapped.
In order to reduce scrap caused by defective expansion vessels, the expansion vessels are changed after a set number of cycles. However, this preventative measure does not always prevent scrap because a pinhole leak may develop prior to this set number of cycles. Additionally, this preventive measure may change out some expansion vessels that still have many more cycles in their effective life. Thus, scrap tires may still result, money is lost by not utilizing the full life of the expansion vessel, and additional labor (costs arise due to more frequent expansion vessel changes.
Curing device manufacturers have attempted to limit scrap products caused by pinhole leaks in expansion vessels by incorporating a monitoring system into the curing device. Before a product to be cured is placed into the curing device, the expansion vessel of the curing device is pressurized. The pressurized expansion vessel is isolated by closing the valves in the supply and discharge lines. The monitoring system monitors the pressure in the expansion vessel to determine if a leak is present
U.S. Pat. No. 5,417,900 entitled "VACUUM LEAK DETECTOR FOR A TIRE CURING PRESS" discloses a device and a method for detecting a leak in an inflatable elastomeric bladder of a curing press. The spent curing fluid is pumped through an exhaust line in which a venturi ejector is located. The venturi ejector causes a vacuum to be created as the fluid is being removed. A vacuum sensor monitors this vacuum. If a predetermined vacuum strength is not reached in a predetermined time, a leak is assumed to exist.
These prior art systems work well when a leak occurs at the coupling attaching the expansion vessel to the curing device. However, these systems are not adequate for detecting a pinhole leak in an expansion vessel. Since the quantity of fluid lost through a pinhole leak is small, relative to a coupling leak, determining the presence of such a leak using pressure and/or vacuum sensors is very difficult. These devices are also ineffective because they can also signal leaks in bladder seals, thus producing false positive responses.